Readily polymerizable ethylenically unsaturated aromatic compounds, such as styrene monomer, are important chemicals of commerce, the current production of styrene monomer being about 10 billion pounds per year. Essentially all of the styrene currently produced is made by processes involving the dehydrogenation of ethylbenzene. Such processes typically include a styrene distillation step wherein the distillation is conducted under a nitrogen atmosphere at a temperature of about 110.degree. C. and a pressure in the range of about 100 to 190 mm Hg. Under these conditions, the reactivity of styrene necessitates the use of a relatively large amount of process inhibitor to prevent polymerization. Likewise, current processes for the production of other ethylenically unsaturated aromatic compounds, such as .alpha.-methylstyrene, vinyl toluene, vinyl naphthalene, divinylbenzene, etc., commonly employ vacuum distillation techniques and require the use of a process inhibitor.
The industry currently employs 4,6-dinitro-orthocresol (DNOC) as a process inhibitor. However, DNOC is a highly toxic process inhibitor. The toxicity of DNOC raises exposure concerns with regard to monomer production, the preparation of DNOC stock solutions, and the ill effects of accidental spills. Therefore, there exists a need in the industry for a process inhibitor to replace the highly toxic DNOC.
It has now been found that 3,5-dinitrosalicylic acid (3,5-dinitro-2-hydroxybenzoic acid; DNSA) and certain derivatives and isomers thereof can be safely employed as process inhibitors for readily polymerizable ethylenically unsaturated aromatic compounds. DNSA exhibits an effectiveness equal to or greater than DNOC and has been shown to be much less toxic than DNOC. The claimed derivatives and isomers of DNSA have also been found to be effective process inhibitors.
Foord et al. disclose in J. Chem. Soc., 1940, pp. 48-56, the effectiveness of quinones as a class of styrene polymerization inhibitors. The reference further discloses the usefulness as polymerization retarders of aromatic compounds having a number of various substituents, including quinonoid, nitro, phenolic hydroxy, amino, and nitroso groups. However, the reference does not disclose the significantly greater inhibitory effect of DNSA over that observed for the other members of the described class of "retarders".
Japanese Pat. No. 39-1817(1964) discloses stabilizers for use in conjunction with unsaturated polyester resins. The patent discloses that suitable stabilizers are nitro compounds which are represented by formula I. It is further disclosed that 3,5-dinitrosalicylic acid (DNSA) is a suitable stabilizer (see Example 5 of the reference). However, the disclosure of the reference relates solely to the curing of unsaturated polyester resins. Such a system relates solely to a copolymerization process involving different reaction rates from those encountered in the polymerization of styrene, which is a homopolymerization process and to which the present invention relates. In addition, the "stabilizers" of the Japanese patent are utilized in the unsaturated polyester resin in a storage environment, whereas in the present invention, DNSA and the claimed derivatives and isomers thereof are utilized as process inhibitors. In addition, the improved effectiveness of DNSA and certain derivatives and isomers thereof as styrene polymerization inhibitors is not disclosed.
Thus, the improved effectiveness of DNSA and certain derivatives and isomers thereof as polymerization inhibitors was neither disclosed nor suggested by the prior art. Furthermore, the art did not recognize the toxicity problems posed by certain previously known inhibitors and certainly did not propose the use of DNSA and derivatives and isomers thereof as a means for solving those problems.